[Bug libstdc++/94268] std::filebuf is extremely (at least 10x) slow on windows compared to Linux. Even much slower MSVC STL with terrible ABI.

[euloanty at live dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94268

--- Comment #10 from fdlbxtqi <euloanty at live dot com> ---
What about adding another check when BUFSIZ is smaller than 4KB? If it is
smaller than 4kb, adjust the filebuf size to 4kb at least.